JPMorgan Chase & Co, the biggest U.S. bank by assets, said on Wednesday it is being investigated by civil and criminal divisions of the U.S. Department of Justice over offerings of mortgage-backed securities.

The civil division gave the company a notice in May that it had preliminarily concluded that the firm violated federal securities laws in offerings of subprime and Alt-A residential mortgage securities during 2005 to 2007, JPMorgan said.